1. Field of the Invention
The present invention pertains to a pyroelectric detector array where the detector elements are arranged in a circle or other geometric shape. An optical filter made of plastic or other suitable material is mounted directly above the ring of detector elements so that individual optical filter elements of different passbands can be mounted directly above and in proximity to the detector elements.
2. Description of the Prior Art
With increased industrialization, the detection, identification and measurement of gases is becoming more and more important. Air pollution is one area of concern. Other fields requiring gas detection and monitoring include natural gas transmission and distribution, bio-medical instrumentation, and various processes which require both safety equipment as well as process monitoring and control. Chemical information obtained continuously in real time can lead to improved product quality. Further, continuous gas concentration measurements can lead to real-time information on other gas properties, for example, the energy content of natural gas.
Optical techniques are especially suited for many of these applications. Many gases can be uniquely identified by their optical absorption signature when taken over a wide wavelength range. In a mixture of gases, the individual absorption peaks can be measured, or in the case of overlapping spectra, the proportion of each gas can be calculated from prior knowledge of the absorption strengths. Optical techniques can also be truly on-line and easily adapted to remote sensing techniques when necessary. In the optical spectrum, the infrared range is well-suited for absorption measurements and instruments based on absorption, since many gases have their absorption bands in this region. The advantages of an optical absorption technique can often be utilized to the fullest only if the entire wavelength range of interest is available for study.
Several manufacturers fabricate single element pyroelectric detectors. These are housed in transistor type packages such as TO-5 or TO-8. Wavelength discrimination for gas detection and measurement is via the optical filter installed in the cover. Detector arrays which are currently on the market are typically linear arrays (for example, Servo Corp. Model 1508 and 1508 VM). These are suitable only for dispersive type instruments which require optical dispersive elements which can be bulky, have reduced optical throughput, and be expensive for the long wave infrared region. The prior art pyroelectric detector arrays are not suitable for incorporation into gas monitoring, leak detection, and process control instruments.
More specifically, FIGS. 1 and 2 are electrical schematics of single element pyroelectric detectors 100 and 101, operating in the voltage and current modes, respectively. The single element pyroelectric detector includes a specially processed lithium tantalate (LTO) crystal 102 coupled with either voltage mode or current mode preamplifiers. In the voltage mode, a low-noise, low gate-leakage current JFET impedance buffer 103 is used, which produces a low impedance output signal capable of being interfaced to standard electronic circuitry outside the package. In the current mode, as illustrated in FIG. 2, an integral hybridized preamplifier 104 is used which converts the current produced by the crystal 102 to an output voltage. The frequency response is determined by the crystal capacitance and the high value resistor 105. In the voltage mode, as shown in FIG. 1, the resistor 106 is placed in parallel with the crystal, whereas in the current mode, the resistor 105 is placed in the feedback loop of the operational amplifier 104. FIGS. 1 and 2 show the basic configuration of these two operating modes: C.sub.d is the crystal capacitance modeled in parallel with the crystal as a current source, R.sub.i and R.sub.f are the high value resistors, and C.sub.s is the stray capacitance.
The pyroelectric detector 100 or 101 operates on the principle that a change in temperature of the pyroelectric element upon irradiation causes a change in the charge on its surface. The change in charge is related to the temperature change by: EQU .DELTA.Q=.rho.A .DELTA.T.sub.x
Where .rho. is the pyroelectric coefficient, A is the detector area, and .DELTA.T.sub.x is the change in temperature of the pyroelectric element.
The actual measurement made with the circuits shown in FIGS. 1 and 2 is the rate of change of charge or the pyroelectric current. The detectors 100, 101 therefore measure the rate of temperature rise. They are thermal detectors as contrasted to photon detectors which have a band gap mechanism for photon detection. The spectral response is determined by the nature of the top electrode or the absorptive coating applied to the top electrode. For wideband spectral absorption and increased sensitivity, the top electrode is usually blackened using black paints, inks, evaporated black coatings or other selective absorbers. Since the detector is thermal in nature, the nature of the crystal mount and the rate of heat drain from the crystal is important in determining the device characteristics.
Lithium tantalate (LTO) is the preferred pyroelectric material for many applications since it is inert, rugged, with a high Curie temperature (610.degree. C.), provides an adequate signal-to-noise ratio and is stable over temperature. The single element detector usually consists of the sensing element, JFET buffer or op-amp, resistor and IR filter window integrated into a small hermetically sealed package, typically a TO-5 style transistor package which also affords a high degree of electrostatic shielding. The single element detectors can be built in production quantities for commercial products, or built and qualified to high reliability requirements for use on satellites for earth sensor applications or special purpose military systems.